First off; if things are broken, fix them first. Adding extra partners to a fucked-up painful and unhealthy situation doesn't ever help things, it just makes the bad things more complicated.

As far as poly goes; Would YOU date you/her in your current situation?

Think about it as a pickup line from a 3rd party. Just saying, if it sounds bad to you, how would it sound to someone else?

This is indeed another important aspect to consider. Whomever either of you get involved with would have an enormous burden placed on them, as you would both be hoping that they would magically make things better for you. No one in their right mind would want to take that on. Everyone wants to enter into healthy situations where their needs would be met, not where they had to navigate around two people fighting in a fucked-up dynamic.